(a) Field of the Invention
The present invention relates to an apparatus for measuring a trench depth and, more particularly, to an apparatus for measuring the depth of a trench, groove or hole (collectively referred to as xe2x80x9ctrenchxe2x80x9d in this text) formed on a semiconductor wafer etc. The present invention also relates to a method for measuring the depth of such a trench.
(b) Description of the Related Art
Along with development of higher integration of semiconductor devices, a three-dimensional structure is now employed for arranging semiconductor elements, in addition to the conventional two-dimensional structure. In the three-dimensional structure, as well as a two-dimensional structure, a trench is generally formed for electrically isolating adjacent semiconductor elements. Since the depth of the trench significantly affects the device characteristics of the semiconductor elements, it is desired to measure the depth of the trench thus formed or examine whether or not the trench has a design depth.
A technique using an optical interference or optical diffraction has been generally used for measuring the depth of a fine trench in a semiconductor device. However, a measuring apparatus using such a technique has a complicated structure and thus is expensive. In addition, such a measuring apparatus is not suited for measuring the depth at a high speed
FIG. 1 shows a conventional apparatus for measuring the depth of a trench, described in Patent Publication JP-A-61-099806. Laser beam 111 emitted from a laser source 11 is focused by a lens 12 to be incident onto an end of an optical fiber 13. The laser transmitted by the optical fiber 13 is received by an optical head 14, which collimates the laser beam to form a parallel ray 112. The parallel ray 112 is incident onto the sample 17 having a subject trench 10 to be measured for the depth thereof.
The optical head 14 then receives the laser reflect from the sample 17, and reflects the received laser by a semi-transparent mirror prism in the optical head 14 to be incident onto an optical sensor 18. The optical sensor 18 converts the laser to an electric signal, witch is measured by a volt meter 19 to detect the intensity of the reflected laser. A power supply source 20 supplies power to the optical sensor 18.
A slider 21 attached onto the optical head 14 is moved along an arched arm 22 having a scale thereon for changing the incidence angle of the laser 112 while observing the indication of the volt meter 19. The depth of the trench 10 can be measured from the incidence angle xcex8R at which the reading of the volt meter 19 assumes a minimum. The depth (d) is obtained from the following equation:
xcex8r=tanxe2x88x921(a/d)
wherein xe2x80x9caxe2x80x9d is the width of the trench.
In the technique as described above, it also takes a long time to measure the depth of the trench and not suited for measuring the depth of a subject trench at a high speed.
In view of the above problems in the conventional techniques, it is an object of the present invention to provide an apparatus and a method for measuring the depth of a subject trench at a higher speed with a simple structure of the apparatus.
The method of the present invention for measuring the depth of a subject trench, in one embodiment thereof, includes the steps of forming, on a substrate, a plurality of measurement trenches having different widths ai(ai less than ai+1, i=1, 2, . . . , n) and a depth equal to the depth d0 of the subject trench, irradiating the measurement trenches with a parallel ray at an incidence angle of xcex8 with respect to the surface of the substrate at the location of the trenches and detecting the reflected ray from tie substrate, separating the measurement into a first group of trenches providing dark images and a second group of trenches providing bright images, and calculating the range for the depth d0 by using the largest width ak of the trenches in the first group and the smallest width ak+1 of the trenches in the second group based on the following relationship:
(ak/2)tan xcex8 less than d0xe2x89xa6(ak+1/2)tan xcex8. 
By setting the incidence angle xcex8 of the parallel ray with respect to the substrate surface at 45 degrees in the above method, calculation can be simplified due to the fact that tan xcex8=1. This further reduces the time length for the measurement. In addition, by determining the term {(ai+1xe2x88x92ai)/2}tan xcex8 below an allowable error for the depth, the accuracy of measurement based on the above relationship can be improved
The method of the present invention, in another embodiment thereof, includes the steps of preparing a plurality (M) of reference image patterns each formed from a plurality (N) of reference trenches having different widths of aj(aj  less than aj+1, j=1, 2, . . . N) and a common depth selected from M different depths of dh (dh less than dh+1, h=1, 2, . . . M, and Mxe2x89xa0N or M=N) by using a parallel ray irradiated at an incidence angle of xcex8 with respect to the surface of a reference substrate having the reference trenches, forming, on a measurement substrate, a plurality of measurement trenches having different widths ai, (ai less than ai+1, i=1, 2, . . . , n) and a depth equal to the depth d0 of the subject trench, irradiating a parallel ray at an incidence angel of xcex8 with respect to the surface of the measurement substrate and detecting the reflected ray from the measurement substrate, detected a boundary width (or critical width) between the widths of a first group of trenches providing dark images and the width of a second group of trenches providing bright images, and selecting, for the depth d0, the depth of the plurality of reference trenches which provide one of the reference image patterns having a boundary width corresponding to the boundary width detected from the measurement trenches. The depths dh of the reference trenches are measured beforehand by using a known technique.
The apparatus of the present invention for measuring the depth of a subject trench, in one embodiment thereof, includes a device for forming on a substrate a plurality of measurement trenches having a depth equal to the depth of the subject trench, a light source for irradiating light to the substrate at an incidence angle of xcex8 with respect to the substrate surface, a CCD camera for detecting the images of the trenches in the reflected light from the substrate to output a image signal, and a processing unit for calculating the depth d0 of the subject trench based on the image signal from the CCD camera, by separating the measurement trenches into a first group of trenches providing dark images and a second group of trenches providing bright images, and calculating the range for the depth d0 by using largest width ak of the trenches in the first group and the smallest width ak+1 of the trenches in the second group based on the follow relationship:
(ak/2)tan xcex8 less than d0xe2x89xa6(ak+1/2)tan xcex8.
The apparatus of the present invention for measuring the depth of a subject trench, in another embodiment of includes a storage device for storing a plurality (M) of reference images patterns each formed from a plurality (N) of reference trenches having different widths of aj (aj less than aj+1,j=1, 2, . . . N) and a common depth selected from M different depths of dh (dh less than dh+1,h=1, 2, . . . M, and Mxe2x89xa0N or M=N), a device for forming on a substrate a plurality of measurement trenches having a depth equal to the depth of the subject trench a light source for irradiating light to the substrate at an incidence angel of xcex8 with respect to the substrate surface, a CCD camera for detecting the images of the trenches the reflected light from the substrate to output an image signal, and a processing unit for calculating the depth do of the subject trench based on the image signal from the CCD camera by detecting a boundary width between the widths of a first group of trenches providing dark images and the width of a second group of trenches providing bright and selecting, for the depth d0, the depth of the plurality of reference trenches which provide one of the reference image patterns having a boundary width corresponding to the boundary width detected from the measurement trenches.
In accordance with the present invention, the depth of the subject trench can be measured by using a parallel ray in a smaller time length with a simple structure of the apparatus without using a optical interference or optical diffraction.
The above and other objects, features and advantages of the present invention will be more apparent from the following description, referring to the accompanying drawings.